Various types of non-impact printing process have been developed because this approach offers attractive features such as increased speed and versatility in printing techniques. One form of non-impact printing process is ink jet printing and involves the modulation of a stream of fluid ink drops which are recorded on a record medium. Various types of ink jets systems presently exist. One such system employs a small nozzle to which fluid ink is delivered under pressure. As the ink exits the nozzle, instabilities due to surface tension forces cause the stream to break up into series of drops. This break up is synchronized by vibrating the fluid which results in uniform drop size and spacing. The rate of drop formation is synchronized with a charging means which induces an electrostatic charge upon each drop as it is formed, the size of the charge being directly related to the input signal voltage. The ink drops with their respective charges then pass through a constant electric field created by a pair of deflection plates which are maintained at a relatively high potential difference. The high electric field causes the ink drops to deflect according to the charge which they carry, with the ink drops then impacting the record medium sequentially (in the same order that they issue from the nozzle) or entering an ink sump for return to an ink reservoir.
Although legible printing results when this basic system is employed, two primary factors, aerodynamic and electrostatic interaction, have deleterious effects on the drops, which alter their inflight paths and result in degraded print quality or drop registration. The first, aerodynamic interaction, occurs as a result of the aerodynamic wake created by each drop as it passes through the air to the record medium. The effect of this wake is to alter the flight path of the drop immediately following because of their close proximity to one another. This flight path alteration is due to the fact that the wake of one drop tends to reduce the air drag of the succeeding drop causing the succeeding drop to remain in the electric field less time than the drop which creates the wake, thus being deflected less. In other words, succeeding drops meet less air resistance.
One approach to the aerodynamics interaction problem is to provide laminar air flows perpendicular to the print access of the ink stream in an attempt to minimize this effect. However, this requires additional engineering and adds to the complexity of the system. Also, an additional force imparted to the ink drops by the laminar air flow which must then be compensated for, thus making this approach to the aerodynamic problem unattractive.
The second factor, electrostatic interaction, whereby two successive drops may be either attracted or repelled by one another in accordance with their respective charges has a similar degrading affect upon the print quality. Once again this is due to the close proximity of the drops, which are printed sequentially. One approach to alleviate this electrostatic interaction has been to buffer the signal carrying charged drops with one or more uncharged drops. For example, every third drop carries a signal charge with the two intervening drops remaining uncharged and directed toward the sump. While drop registration is somewhat improved using this technique, the effect of using one or more drops between charged drops as buffers is to reduce the drop rate and thus the speed of printing. Not only is this undesirable but, aerodynamic interaction is still present with the approach.